The magnetic compass of migratory birds is thought to rely on the radical pair mechanism operating inside a cryptochrome blue-light photoreceptor [1]. It is imperative that the radical pair exists in a non-equilibrium coherent state, long enough for the Earth’s magnetic field to have an influence on the underlying coherent spin dynamics. Several interactions weaken the coherence of the radical pair in a process called spin relaxation [2]. Here, we investigate several dihedral and librational angles in the flavin adenine dinucleotide (FAD) and tryptophan (Trp) radical pair inside cryptochrome from European robin in order to characterise spin relaxation dependent on thermal motion.
We combined all-atom molecular dynamics (MD) simulations, density-functional-theory (DFT) based calculations and spin dynamics calculations. To account for spin relaxation Bloch-Redfield-Wangsness relaxation theory was employed.
Through analysis of cryptochrome dynamics, we have established the contribution of different degrees of freedom to the coherence lifetimes of potential radical pairs inside cryptochrome. This analysis relies on the time-dependent hyperfine interactions for the FAD and Trp radicals, which permitted calculating the quantum yield anisotropy of the radical pair reactions in a magnetic field. The quantum yield anisotropy is a measure for the sensitivity of the birds’ magnetic compass. The compass sensitivity and dynamical parameters were compared for radical pairs in the cryptochrome of European robin and thale cress to conclude if a motion optimization in the bird’s radical pair lead to a significantly better perception of the magnetic field.